Certain triblock copolymers have been known, each of which comprises an acrylic ester polymer block as a central block and other polymer blocks such as methacrylic ester polymers combined at both ends of the central block (Japanese Examined Patent Application Publication No. 25859 of 1995 corresponding to U.S. Pat. No. 5,264,527). In this technique, the triblock copolymers are prepared by anionic polymerization using an organic alkali metal compound or an organic alkaline earth metal compound as a polymerization initiator.
However, this conventional technique is mainly intended to provide a triblock copolymer having a narrow molecular weight distribution in terms of a molecular weight polydispersity (Mw/Mn) of from 1.05 to 2.0 but is not specifically intended to improve mechanical properties and flexibility in, and to impart isotropy to, such triblock copolymers and molded articles obtained therefrom.
The present inventors have found that, by subjecting an acrylic ester and another (meth)acrylic ester monomer different from the acrylic ester in the presence of an organic lithium compound and a specific organoaluminum compound, a block copolymer having a polymer block comprising the acrylic ester and another block copolymer comprising the other (meth)acrylic ester monomer can be produced in a high purity and a high safety with a high block efficiency even without the use of a polar organic solvent, and have filed a Japanese patent application (Japanese Unexamined Patent Application Publication No. 11-335432).
After intensive investigations in consideration of these conventional technologies, the present inventors have found the following findings (i) to (v) relating to a block copolymer having at least two polymer blocks miscible with each other and at least one (meth)acrylic ester polymer block immiscible with the polymer blocks (hereinafter may referred to as “(meth)acrylic block copolymer”):                (i) The (meth)acrylic block copolymer exhibits varying flexibility depending on the content of a polymer block working as a soft segment.        (ii) To obtain a (meth)acrylic block copolymer having high flexibility, the content of a polymer block working as a soft segment may be generally increased. However, surface gluing (adhesion) of the (meth)acrylic block copolymer and articles obtained therefrom increases with an increasing content of the polymer block. Consequently, when the target (meth)acrylic block copolymer having high flexibility is obtained in the form of grains such as pellets or in the form of powders, the grains or powders are stuck together to thereby deteriorate handleability, or the resulting molded article has a tack on its surface or exhibits deteriorated appearance due to attached dust.        (iii) In general, to obtain a molded article having high strength and exhibiting less sticky, the content of a polymer block working as a hard segment may be increased (the content of a polymer block serving as a soft segment may be decreased) to thereby yield a (meth)acrylic block copolymer having somewhat lower flexibility. However, the resulting molded article prepared by melt molding of the (meth)acrylic block copolymer exhibits increasing anisotropy and thereby significantly increases differences in mechanical properties depending on the angle to a melt flow direction of the polymer in melt molding with an increasing content of the polymer block working as the hard segment or with a decreasing content of the polymer block working as the soft segment. For example, the molded article prepared by melt molding of the (meth)acrylic block copolymer having somewhat lower flexibility exhibits elasticity and can thereby sufficiently elongate in a traversal direction to the flow direction of the polymer in melt molding but may exhibit insufficient elasticity and may thereby elongate insufficiently in a direction in parallel with the flow direction.        (iv) A molded article prepared by injection-molding such a (meth)acrylic block copolymer having somewhat lower flexibility as described in (iii) often brings disadvantages such as molding shrinkage, warpage, and distortion.        (v) To obtain a pressure-sensitive adhesive composition having high cohesion (creep property)) by using a (meth)acrylic block copolymer as a base polymer in combination with other additives such as a tackifier, the content of a polymer block working as a hard segment may be increased (the content of a polymer block working as a soft segment may be decreased) to thereby yield a (meth)acrylic block copolymer having somewhat lower flexibility. However, the resulting pressure-sensitive adhesive composition may often exhibit decreased adhesive strength and tackiness. Pressure-sensitive adhesive products such as pressure-sensitive adhesive tapes obtained from the pressure-sensitive adhesive composition exhibit anisotropy in their pressure-sensitive adhesive layer and often bring warpage.        
In addition, the present inventors have found the following problems (vi) and (vii).                (vi) To yield a (meth)acrylic block copolymer having satisfactory elastomeric properties, the glass transition temperature of a polymer block working as a hard segment must be equal to or higher than room temperature. To use the block copolymer as an elastomer at higher temperatures, the glass transition temperature is preferably further higher. Certain methacrylic ester polymer blocks have been conventionally known as polymer blocks to constitute a hard segment. However, it takes a long time to polymerize such methacrylic esters and thereby it takes a long time to prepare such (meth)acrylic block copolymers.        (vii) It takes a shorter time to polymerize acrylic esters than to polymerize methacrylic esters, and these acrylic esters are suitable for the preparation of (meth)acrylic ester block copolymers.However, such an acrylic ester polymer generally has a low glass transition temperature, and the resulting acrylic ester polymer block cannot significantly work as a hard segment. Among such acrylic ester polymers, an isobornyl acrylate polymer is known as a polymer having a relatively high glass transition temperature. However, isobornyl acrylate has a small difference in boiling point with an alcohol contaminated as an impurity during its production and cannot be significantly purified according to a conventional purification process such as distillation, and this type of acrylic esters are not suitable for anionic polymerization.        
Accordingly, an object of the present invention is to provide a (meth)acrylic block copolymer obtained by anionic polymerization and comprising at least two polymer blocks miscible with each other and at least one (meth)acrylic ester polymer block immiscible with the miscible polymer blocks, which block copolymer has a low melt viscosity and exhibits satisfactory melt fluidity, high moldability and coatability, and excellent elasticity and mechanical properties.
Another object of the present invention is to provide the (meth)acrylic block copolymer which has reduced mechanical anisotropy and excellent isotropy and can thereby yield a molded article having mechanical properties not significantly varying between a melt flow direction of polymers in its molding and another direction.
Still another object of the present invention is to provide the (meth)acrylic block copolymer that can yield a target molded article with high dimensional accuracy without disadvantages such as molding shrinkage, warpage, and distortion.
Yet another object of the present invention is to provide a novel (meth)acrylic block copolymer having at least one acrylic ester polymer block as a hard segment.
Another object of the present invention is to provide a process for easily producing the (meth)acrylic block copolymer.
Still another object of the present invention is to provide a molded article that comprises the (meth)acrylic block copolymer, has reduced anisotropy and exhibits excellent mechanical properties and dimensional accuracy.
Yet another object of the present invention is to provide a pressure-sensitive adhesive composition that comprises the (meth)acrylic block copolymer, has a low melt viscosity, exhibits satisfactory processability in a coating process and other processes for the production of pressure-sensitive adhesive tapes and other pressure-sensitive adhesive products and exhibits satisfactory adhesive properties.